1. Field of the Invention
This invention relates to seismic vibrators and, more specifically, to a system for monitoring and controlling the hold-down weight of a seismic vibrator.
2. Brief Description of the Prior Art
The currently used seismic p-wave vibrators utilize an actuator to generate a force that is imparted through a baseplate or pad in contact with the surface of the earth. The weight of the vehicle upon which the actuator is installed is used to provide a static force load on the baseplate. If inadequate static force (hold-down) is provided, the baseplate will lose contact with the earth and signal to noise level degradation will occur as a result thereof because harmonic noise is produced when the baseplate loses such contact. This phenomenon is called decoupling. In most vibrators used today, the hold-down force is applied to the baseplate through a system of airbags. The airbags act as acoustic isolators, their low spring rate in conjunction with the mass of the vehicle forming a mass/spring system which has a low resonant frequency, typically less than 2 hertz. The airbag system thus acts as an acoustic isolator, permitting the static force provided by the vehicle weight to be applied to the baseplate but blocking forces generated by the actuator (sweep frequencies are greater than 5 hertz typically) from acting on the vehicle frame.
The vibrator is also equipped with a lift system for raising and lowering of the baseplate. When a sweep is to be generated, the baseplate must be in contact with the earth. The vehicle is jacked up so that as much of the vehicle weight is applied to the baseplate as possible, the wheels generally still contacting the earth. Often, on truck mounted actuators, two or more wheels may actually lose contact with the earth when the vehicle is jacked up.
A sweep interlock system is implemented on all systems currently in use. This interlock system requires the pad (baseplate) to be down before a sweep can be initiated. The interlock systems in current use determine this condition by monitoring the position of the pad with respect to a preset location relative to the vehicle frame or the pad is always lowered to its maximum extension until a mechanical stop is reached. From the standpoint of maximum time savings, it is best to not lower the pad any more than required to achieve adequate hold-down. It takes time to raise and lower the pad (this operation may exceed 1500 times per day). The less stroke used, the less time spent for this operation.
In uneven terrain, the extension required to achieve adequate hold-down is highly variable. For example, if the pad is positioned over a hole but the truck wheels are on high ground, the baseplate must be lowered considerably further to achieve adequate hold-down than in a situation where the baseplate is located over a mound. Also, the current interlock system may indicate the pad is down even though the pad is not in contact with the earth if the pad is over a depression.
A further problem with prior art systems is that the baseplate moves rapidly into the hold-down position, often making forceful contact with the earth and setting up impulses that are later picked up during a seismic test. Also, upon raising the pad after a sweep during the recording time, rapid pad movement can also be a source of unwanted seismic noise as is the noise generated when the vehicle tires come in contact with the earth.